The present invention relates to the field of coating dies and, in particular, to a methodology for the design of heated coating dies which are capable of maintaining dimensional flatness of its coating lips at operating temperature under actual operating conditions.
A heated coating die is typically used to coat molten polymer containing materials, such as adhesives and other coatings (collectively “coatings”). These coatings are fed into the coating die, which distributes them across its width. Pressure forces the coating fluid through a feed gap formed in the die. The exiting point of the gap is referred to as the coating lips. In many coating applications, the lip faces form a film on the substrate at the lip faces. This film forming region is referred to as the coating bead. In order for the final coating to be uniform across the width of the coating lips (and thus the coating), the coating lips and substrate need to form an even gap (assuming the distribution within the die is uniform).
Lip face flatness measurements on commercially available heated coating dies indicate that the lip surfaces when heated are far from flat. Though the coating lips may be ground to better than 0.001″ when cold, the state of the die when heated can be bent several thousandths of an inch. This does not lend itself to a robust coating process. Three known methods of managing the bending state are:
(1) Attempt to bend the die in the opposite direction mechanically, typically by using adjustments associated with the die station.
(2) Machining the coating lips flat while the die is heated as part of the fabrication of the die.
(3) Pushing the coating lips and substrate into a soft rubber roller, then use feed gap adjustments to redistribute the coating fluid to counteract the uneven flow resistances across the lip.
Though all these methods are in use, none of them lead to a sufficiently controlled and robust process.
In the first method, the loss of precision in the die is transferred via mechanical forces to another device (i.e., die station), which then loses its precision. Additionally, internal stresses which cause the bending are not eliminated, but rather shifted. Finally, once coating starts, the bending state can change due to interaction of the die's heating system and the flow of coating fluid, making the initial adjustment ineffective.
The second method also develops problems. First, even if the die can be machined while heated, when the die is cold it will be bent in the opposite direction which creates uncertanties in its mounting to the die station. Additionally, once coating starts, the bending state may change leading to the machined surface no longer being flat. Further, there is uncertainty as to how flat a die can be machined while hot.
The third method is highly non-linear and can lead to long unstable start-ups of the production line. It can also lead to defects in the coating, which may not be discovered in a timely manner.
All three of these methods suffer from the difficulty in determining the initial hot gap between the coating lips and substrate. In a common methodology a light is shone through the gap between the lip face and substrate (or back-up roll), and the die is visually adjusted to be parallel. If the evenness of this gap changes significantly at start-up due to the interaction of the heating system and flow of coating fluid leading to a temperature redistribution within the die and thus the bending state changing, another uncertainty is thereby added to the process.
A need therefore exists for a robust, quick start-up coating process, which is stable before and during coating, and in which the bending state is controllable. The present invention provides a solution to meet such need.